Yes, me too, several times. Sometimes for ambiguous signs, sometimes just carelessness. Some awkward moments when working in mainland Europe where a lot of restrooms are mixed - there's always that level of uncertainty - is that a mixed restroom or am I not seeing an obscure sign somewhere - dither dither, decide to go in as no one's about - then emerge from the cubicle to the hand washing area to find it full of men milling about. When it's happened, I've not had any odd looks so have assumed either they're very polite to confused foreigners or it's a mixed room.